For safe lift-off, flight and landing, the mass and center of gravity of space vehicles, as well as of aircraft generally, must lie within predetermined ranges unique to each vehicle. To assure that these ranges are not exceeded, the mass and center of gravity of a vehicle is first determined while the vehicle is in an unloaded state. Then, the mass and center of gravity is determined for each item of cargo and a manifest is prepared for a balanced distribution of the cargo within the vehicle. Recent techniques for determining the mass and center of gravity usually require that each item be separately placed on a tilting type table, and then either weighed in several positions or balanced in a three-coordinate jig.
As the cargo carrying capacity of space vehicles increases, this procedure became excessively time consuming. With the advent of the Shuttle type space vehicle, it became possible to assemble the cargo into individual lots aboard pallets and then place the loaded pallets aboard the vehicle. Pallets usually have four trunnions that extend beyond the sides of the pallet in a single horizontal plane. Although pallets and their transportation frames differ in shape and construction so as to conform to particular or unique items of cargo, the size, spacing and location of the pallet's trunnions are standardized to facilitate stowage of pallets during preparation for flight.
Loaded pallets are too bulky and heavy to be weighed on tilting tables or jigs and must, therefore, be raised by a pair of large overhead cranes having electrical load cells attached to its spreader bars. Then, if the mass or center of gravity of the pallet is found to be unacceptable, either some cargo is removed from the pallet or the pallet is unloaded and the cargo reloaded to provide an acceptable distribution of mass within the pallet. This is an overly time consuming and expensive procedure because each pallet is typically carefully loaded with unique items of cargo such as precision scientific instruments at research laboratories or other distant points of origin usually lacking such facilities as overhead cranes for weighing the loaded pallets, and then packed in a special transportation frame to protect the cargo against vibration and shock. The loaded pallet and its transportation frame is subsequently shipped to a launch site where the pallet is completely removed from its transportation frame and weighed. If weighing indicates that the loading of the pallet is unacceptable, the pallet must be returned to its point of origin, unpacked, and reloaded, an expensive and time consuming requirement.
Recent suggestions for alleviating the expensive and inconvenience of this requirement have included the installation of a pair of overhead cranes of adequate capacity at each point of origin. Such suggestions, however, are impractical because of the cost of both the cranes and their associated facilities, and unfeasible because many points of origin may load pallets only infrequently. Furthermore, even when available, dual overhead cranes which are, by nature, stationary installations, require that pallets be both loaded and weighed within a single area in close proximity to the cranes, thereby preventing both dispersion of the pallets and cargo to decentralized locations more convenient to the responsible personnel and use of the cranes and their associated facilities for other purposes during loading and weighing of a pallet. Moreover, weighing loaded pallets with overhead cranes is not a particular reliable procedure because the spreading bars and slings used with overhead cranes to weigh loaded pallets frequently cause the pallet to swing or sway during lifting which causes a preliminary distribution of cargo to shift within the pallet, thereby necessitating rearrangement of the cargo. Also, unexpected swinging or swaying can cause damage to the cargo as well as injury to attendant personnel.